The invention pertains to an angle of rotation sensor, wherein the rotational position of a permanent magnet is evaluated by at least one Hall element. In sensors of this type, the rotational position of the body, the angular position of which needs to be determined, is coupled with at least one permanent magnet. If the rotational position is changed, the distribution of the corresponding magnetic field is also changed relative to a stationarily arranged Hall element. Consequently, the angular position of the body that is coupled with the permanent magnet can be determined based on the voltage measured on the Hall element.
U.S. Pat. No. 4,829,248 discloses a rotational body in which a series of permanent magnets is arranged on its outer circumference. If this body rotates, the intensity and the direction of the magnetic field that penetrates two Hall elements is changed, wherein the Hall elements are stationarily arranged opposite to the rotating body in the vicinity of the outer circumference. One disadvantage of this measuring arrangement can be seen in the fact that it requires a large number of permanent magnets. In addition, this measuring arrangement is designed merely for evaluating the relative movement between the body surface and the Hall probe. Consequently, an absolute measurement of the position of the body within a small angular range is not possible with the measuring arrangement disclosed in U.S. Pat. No. 4,829,248. U.S. Pat. No. 5,325,005 discloses a synchronous motor, the rotational field of which is controlled as a function of the rotational position of the armature. In this case, the Hall probes serve as triggers for advancing the rotational field.
In known sensors for angles of rotation which operate with Hall probes, a value for the angle of rotation of approximately 180xc2x0 lies between the positive and the negative maximum of the measured magnetic flux, and consequently of the voltage delivered by the Hall probe. If the measurement is carried out over a range that exceeds 180xc2x0, the measured values may be ambiguous. For an angular value that exceeds 180xc2x0, a definitive measurement result can be obtained by utilizing two or more Hall probes. However, this method requires a larger number of Hall probes, i.e., the expenditure for the corresponding evaluation is also increased. The invention aims to disclose an arrangement that makes it possible to measure the angle of rotation over an angular range that by far exceeds 180xc2x0, namely with the least possible technical expenditure and, in particular, a small number of Hall probes.
The invention, in principle, proposes to provide the rotatable magnet with a yoke that essentially surrounds the magnet in circular fashion, and to design and/or rotationally arrange the magnet (with or without pole shoes) within the yoke in such a way that it delivers definitive measuring results over a measuring range that by far exceeds 180xc2x0. In this context, the term yoke refers to a stationary, magnetically conductive body that surrounds the magnet and forms a path for the magnetic flux emerging from the magnet which has a superior magnetic conductivity. In this respect, the yoke acts similarly to the stator of an electric motor. However, the yoke simultaneously fulfills a shielding function with respect to possible external electromagnetic interference fields. In this case, the yoke does not necessarily have to be designed circularly. The yoke may also have a different shape as long as it is able to increase or keep the angular measurement range of the angle of rotation sensor large. The yoke may, for example, have an elliptical inner contour. However, it is essential to the invention that all possible measures which produce the largest possible angular range between the measured maximum and the measured minimum of the magnetic flux be combined with one another, wherein the angular range to the next maximum is correspondingly shortened within the adjacent measuring range because the original position of the angle of rotation sensor is naturally reached again after 360xc2x0.
Consequently, the measures according to the invention consist, in principle, of designing the angle of rotation sensor in an asymmetric fashion such that an angular range is formed in which the maxima lie as far apart from one another as possible. In a particularly simple measure for achieving this objective, the magnet is arranged to be offset relative to its rotational axis. In addition, the rotational axis of the magnet may also be arranged to be offset relative to the rotational axis of the body, the rotational movement of which needs to be measured. However, this is not necessary. If both axes are aligned with one another, a simpler mechanical construction is attained such that the magnet and the body, the rotational movement of which needs to be measured, can be arranged on a common axis. The mechanical construction can be additionally simplified by aligning the central axis of the yoke with the rotational axis of the magnet which may also be aligned with the rotational axis of the body to be measured. With respect to the magnet, it can be generally stated that this magnet may be magnetized throughout or provided with pole shoes that direct the magnetic flux of the core magnet to the yoke via an air gap in suitable fashion within the yoke space. In order to maintain the magnetic resistance for the magnetic flux at a minimum, the air gap between the magnet and the yoke or between the pole shoe and the yoke, respectively, should be maintained as small as possible over the entire angle of rotation. With respect to the offset position of the magnet relative to its rotational axis, the magnet may be arranged in such a way that its flux within the magnet core extends radially to the contour of the adjacent yoke region or tangentially thereto. These indications may also refer to the rotational axis of the magnet, i.e., the inner magnetic flux either extends radially to the rotational axis or tangentially thereto. In the present embodiment, a radial orientation of the magnetic flux is proposed. However, if deemed practical with respect to the other components of the angle of rotation sensor according to the invention, it is also possible to choose a tangential internal magnetic flux or to define an intermediate position for the magnet which lies between the radial and the tangential arrangement.
If the magnet is arranged to be radially offset from its rotational axis and a radial internal magnetic flux is chosen for the magnet core, one realizes an embodiment which is characterized by a high flux density within the yoke.
A particularly simple mechanical arrangement is defined by a body, the contour of which is arranged to be circularly symmetrical within the yoke, but causes a highly asymmetric magnetic field to result which contributes significantly to improving the desired result.
In a particularly simple design for a magnet that preferably does not have pole shoes, the magnet has an essentially circular or annular cross section. If the magnet has an annular cross section, the magnetic field is preferably aligned by providing the ring with two radially magnetized regions which oppose one another, wherein one region extends over a much larger angle than the other region. Due to this measure, the desired asymmetric distribution of the flux over the yoke circumference is achieved with a correspondingly asymmetric distribution of the radially extending field within the air gap between the magnet or pole shoe and the inner contour of the yoke.
Another principle for attaining the desired asymmetry is defined by an arrangement which contains a permanent magnet that may be arranged centrally with reference to the rotational axis of the magnet or radially offset thereto. This permanent magnet usually has only one continuous magnetization that extends to one side. In order to attain or improve the desired asymmetry of the magnetic flux or adapt this asymmetry of the magnetic flux to the corresponding requirements, the permanent magnet is provided with pole shoes that are preferably realized in the shape of sectors of a circle. In order to attain, improve and adapt to the corresponding requirements of the desired asymmetry, the pole shoes extend along the contour of the yoke over a different angular range. Alternatively or additionally thereto, the outer surface of the pole shoe, which is situated opposite to the inner contour of the yoke, may be provided with a different curvature such that the width of the air gap between the pole shoe and the inner surface of the yoke changes over the circumferential angle.
The measuring range can, as initially mentioned, be increased by utilizing two or more Hall probes. A particularly sensitive measurement can be achieved in the case where the Hall element or the Hall elements, which are preferably offset relative to one another by 90xc2x0, are inserted into the yoke in such a way that they intersect the magnetic flux that extends tangentially at this location. The design of the yoke should, at least within this region, be chosen such that the entire magnetic flux intersects the surface of the Hall element. However, it is not absolutely imperative to radially insert the Hall element into the yoke. In this respect, it is also possible to insert the Hall element or the Hall elements in such a way that the magnetic flux flows radially through the Hall element(s), e.g., as described in U.S. Pat. No. 5,325,005. This means that the Hall elements are stationarily arranged in front of the magnetically conductive yoke, preferably in the air gap.